This invention relates to an image forming apparatus and method for forming/recording an image while maintaining the quality of a picture by converting resolution of image data.
Printers have recently been given a color printing capability and have come to be utilized by users and means for producing a variety of expressions. In particular, color page printers using electrophotography have become the focus of attention owing to their high-quality printing and high-speed printing capabilities.
A full-color, laser-beam printer, which is one type of color page printer, records a multiple-value colored image by a first process which involves performing a first development by causing a beam to scan a photosensitive drum in a main-scan direction, and then transferring the image to a recording medium such as recording paper on a transfer carrier, and by successive second, third and fourth processes that follow the first process.
An electrophotographic color-laser beam printer is known in which a color image is obtained generally by these four processes using Y (yellow), M (magenta), C (cyan) and K (black) color toners. Recent years have seen the practical application of printers in which halftone image data is received from a host computer as multiple-value information-signal data composed of, say, eight bits per pixel, without the data being binarized as by dithering. These printers output each pixel by printing it in multivalued form. Printers having a printing resolution on the order of 300 dpi (dots per inch), 600 dpi, etc., have been developed and manufactured.
FIG. 46 is a block diagram illustrating the construction of a color laser-beam printer according to the prior art. As shown in FIG. 46, a host computer 201p connected to a printer 202p sends image information, which has been created by application software or the like, in the form of a command or data to a controller 203p within the printer 202p via an interface (not shown).
While expanding the entered command or data to multiple-value (e.g., eight bits) bit data corresponding to pixels of 600 dpi in conformity with the command or data, the controller 203p sequentially plants the data as 600 dpi multivalued data in a buffer memory 108p and stores one page of data. In dependence upon the process timing of a printer engine 207p having a resolution of 600 dpi, the controller sequentially sends a 600 dpi multiple-value image signal to the printer engine to drive the engine in conformity with the image signal, thereby obtaining a color image.
In recent years, information storage devices, such as laser-beam printers, employing electrophotography have come to be used widely as output devices for computers. Owing to the advantages of high printing quality, silence and high speed, these information storage devices have led to rapid growth in the field of desktop publishing (DTP).
Furthermore, electrophotographic color printers have also been developed. Owing to improvements in the capabilities of components such as a controller, which is the image generating unit in a host computer or printer, these color printers handle not only monochromatic printing but also the printing of color images. These printers have been put into practical use and are gaining popularity.
Depending upon the type of color printer, several methods such as the dither method, density-pattern method and error-diffusion method are available as methods for printing a full-color image exhibiting tonality. A characterizing feature of a laser-beam printer is that resolution in the main-scan direction can be changed with comparative ease. For example, laser-beam printers employ pulse-width modulation (so-called PWM) in which grays can be expressed by changing the driving pulse width of a laser diode in dependence upon the value of image data.
In recent monochromatic page printers, the general trend has been to improve picture quality by introducing resolution improving techniques such as smoothing processing for detecting and smoothing the edges of characters and figures. In addition, high-resolution printers in which the resolution of the printer engine serving as the printing mechanism is on the order of 480 or 600 dpi have appeared on the market in place of those having a resolution of 240 or 300 dpi, which was the old standard. By incorporating the aforesaid smoothing processing technique in these high-resolution printers, printing quality has been improved remarkably in comparison with the conventional printers.
Accordingly, there is a need to improve the printing quality of color printers as well by applying resolution-improving processing to smoothen the edges of characters and figures.
In the printers mentioned above, however, the following problem arises owing to the enormous amount of information carried by the signals handled:
Specifically, image information possessing grayscale information for each and every pixel contains an extremely large quantity of information, namely (number of pixels).times.(number of grayscale bits). In this case, the image information must be transferred in conformity with the process speed of the printing process employed by the printer. In addition, an operation is necessary for transforming information indicative of the entered color space to gray-level information of the colors Y (yellow), M (magenta), C (cyan) and K (black).
In order to implement this, an apparatus has been put into practical use in which the printer side is provided with a buffer memory having a capacity equal to or greater than the maximum output size. Once input image information has been stored in the buffer memory, data is transmitted to the printer engine and printed out in accordance with the timing of the printer.
In a printer of this kind, however, memory capacity is very large. Consequently, in a case where resolution is 600 dpi and a multiple-value image of eight bits per pixel is entered to print out data in size A4, the required capacity of the buffer memory is 32 megabytes for each of the colors Y, M, C, K, for a total of 128 megabytes.
This leads to problems such as a major increase in the cost and size of the apparatus. Furthermore, the processing performance of the control unit (CPU) handling the data must be improved. This also is accompanied by higher cost.
Further, in the conventional color printers mentioned above, the image data is multivalued and one picture is reproduced by superimposing images, which are composed of toners of the four colors magenta, cyan, yellow and black, produced from the multiple-value image data. This means that the smoothing processing technique employed in monochromatic printers cannot be applied as is.
Moreover, the above-mentioned conventional color printers often deal with pictorial images such as photographs. This makes it necessary to distinguish between these pictorial images and images such as characters and figures for the sake of processing.
An object of the present invention is to provide an image forming apparatus and method in which memory capacity can be reduced by a wide margin, while a high picture quality is maintained, even when a resolution conversion is performed so as to raise the resolution of image data.
Another object of the present invention is to provide an image forming apparatus and method in which image conversion logic that differs depending upon the characterizing features of an image is applied to effect a conversion of resolution so that a smooth-edged image can be obtained.